Composition and method for forming a protective coating on a zinc metal surface

ABSTRACT

This invention provides a composition and method for forming a protective coating on a zinc metal surface. The composition of the invention comprises an aqueous solution of an inorganic water soluble compound, a metal ion and a salt of an alkanolamine.

United States Patent Houlihan, deceased et al.

[ Dec. 30, 1975 COMPOSITION AND METHOD FOR FORMING A PROTECTIVE COATINGON A ZINC METAL SURFACE Inventors: William F. Houlihan, deceased, lateof Springfield, Mass., by Rosemary Houlihan, executrix; Isaac LairdNewell, Wethersfield, Conn.; Gerald E. La Cosse, Chicopee, Mass.

Assignee: Heatbath Corporation, Springfield,

Mass.

Filed: Mar. 5, 1974 Appl. No.: 448,37 1

US. Cl. 148/6.l4; 148/62; 148/616 Int. Cl. C23F 7/00 Field of Search148/614, 6.2, 6.27, 6.16;

[56] References Cited UNITED STATES PATENTS 3,444,007 5/1969 Maurer etal. 148/616 7/1972 Weaver et a1 148/614 Primary Examiner-Ralph S.Kendall Assistant ExaminerCharles R. Wolfe, Jr. Attorney, Agent, orFirm-Morgan, Finnegan, Pine, Foley & Lee

[57] ABSTRACT 13 Claims, No Drawings COMPOSITION AND METHOD FOR FORMINGA PROTECTIVE COATING ON A ZINC METAL SURFACE Protective coatings areformed on a zinc metal surface by the use of an aqueous solution of aninorganic water soluble compound, a metal ion and asalt of analkanolamine.

BACKGROUND OF THE INVENTION In the prior art, alkaline coatingcompositions have been used for coating zinc and zinc alloys.Compositions for this purpose are disclosed'in U.S. Pat. No. 3 ,444,007.They consist of an aqueous alkaline solution of a metal ion selectedfrom the group consisting of silver, magnesium, cadmium, aluminum, tin,titanium, antimony, molybdenum, chromium, cerium, tungsten, manganese,cobalt, ferrous and ferric with a complexing agent to hold theion insolution. The present invention provides an improvement over the priorart in that none of the complexing agents according to U.S. Pat. No.3,444,007 are required to secure an adherent, corrosion resistant filmon a zinc metal surface. Other improvements are: 1) simplerformulations, 2) case of waste treatment, 3) less costly than employingchelating agents, e.g., sodium gluconate, EDTA, 4) can be formulated asa dry mixture. From the aforementioned patent it can be seen thatwithout the use of one of the disclosed complexers it was not possibleto place a corrosion resistant film on a zinc surface. Note Table I,panel series No. 2. Contrary to the teachings of this patent, applicantshave found that the use of water soluble alkanolamine salt with anadditional inorganic alkaline material will produce a tough adherentcorrosion resistant coating on a zinc metal surface. This is surprisingsince U.S. Pat. No. 3,444,007 discloses only the free base form oftriethanolamine as being useful for developing an alkaline pH insolutions that have one of a recited class of complexers.

As used herein and in the appended claims zinc metal surface -is used toinclude zinc metal objects, zinc coated objects and alloys which containzinc as a principal component.

DESCRIPTION OF THE INVENTION The present invention provides acomposition for coating zinc metal surfaces. The composition may beapplied to a substrate by spraying, immersing, dip-coating or paintingthe object which is to be coated. The coating may be used as a corrosionbarrier or as a base coat that may be used to improve the adhesion ofpaints.

The composition of the invention comprises:

a. an inorganic water soluble alkaline compound;

b. at least one metal ion selected from the group consisting of cadmium,mangnese aluminum, tin, titanium, molybdenum, chromium, tungsten,cobalt, ferrous, ferric and nickel; and

c. a water soluble salt of an alkanolamine in an amount sufficient tomaintain said metalions in solution.

The solvent to be used is water which may include a sufficient amount ofa surface active agent to reduce the surface tension and facilitate thecontacting of the zinc metal surface with the coating solution. Theparticular surface active agent is not critical and anyone may beemployed if it is compatible with the particular 2 metal ions employedat the particular pH level of a given solution.

In general the pH of the solution should be above 7.5. Usually it willbe preferred to use solutions having a pH of from 10.0 12.5. Thoseskilled in the art will be able to select the optimum pH level to beused in a particular system. It is preferred to use the alkali metalhydroxides, such as sodium, lithium or potassium hydroxide to maintainthe desired alkalinity although inorganic alkaline compounds may beemployed such as sodium silicate, potassium silicate, lithium borate,sodium carbonate, potassium carbonate, lithium carbonate or mixturesthereof with or without the alkali metal hydroxides.

Examples of particular metal salts which may be used as a source of ionsinclude nickel nitrate, aluminum chloride, sodium molybdate, manganesenitrate, chromium nitrate, cadmium chloride and cobalt nitrate. Otheruseable salts of the desired metal ion may be selected by those skilledin the art. For example, Langes Handbook of Chemistry, Fifth Ed.,Handbook Publishers, Inc. (1944) at pages 154-263, which is incorporatedherein by reference, discloses numerous water soluble salts of theuseable metal ions that may be employed in the practice of the presentinvention. The water soluble salts of suitable alkanolamines includetriethanolamine hydrochloride; monoethano1-- amine hydrochloride,diethanolamine hydrochloride, triisopropanolamine hydrochloride,aminoethylethanolamine hydrochloride, triethanolamine phosphate,monoethanolamine phosphate, diethanolamine phosphate,triisopropanolamine phosphate and amino ethylethanolamine phosphate.

The compositions of the invention may be prepared as concentrates anddiluted to the optimum concentration prior to use. When diluted for use,it is preferred to use a sufficient amount of an inorganic alkalinecompound to yield a pH in the range of 7.5 13, from about 0.002 to about0.25% by weight of metal ion and more preferably from about 0.01 toabout 0.1% by weight of the particular metal ion. It is preferred toemploy two or more different ions in a given composition. The watersoluble alkanolamine salt is employed at a level which will besufficient to maintain the metal ions in solution. This will vary withthe concentration of the metal ions but it has been found that fromabout 0.05 to about 10% by weight, preferably from about 0.1 to about5.0% by weight may be employed. There does not appear to be anyadvantage in employing an excess of the alkanolamine salt apart fromthat amount needed for the actual complexing reaction. Because of theinherent acidity of some of the water soluble salts of the alkanolaminecompounds, it may be necessary to increase the concentration of thewater soluble inor-' ganic alkaline compound in order to satisfy theabov stated pH requirements. I Combinations of cobalt-iron,chromium-cobalt, cobalt-molybdenum, aluminum-molybdenum, and the likemay be used to obtain advantageous results. The application of thecomposition may be at ambient conditions such as room temperature or atany temperature up to the boiling point of the particular composition.Preferred conditions enabling the formation of the coating involve theuse of the treating solution at a temperature of from 25C to about 88Cfor a time of from about 2 seconds to about seconds. Satisfactorycoatings are obtained in about 10 to 15 seconds and at a temperature ofabout 38C to 71C. Very acceptable coatings may also be obtained inshorter times with solutions of higher alkali and metal ion strength,i.e., higher concentrations and/or higher temperatures. In general,slightly longer times are required for immersion applications but thecomposition of this invention is equally adaptable to spray or rollcoating applications with the appropriate modifications inconcentration, time and temperature.

The appearance of the complex-oxide coating formed on the surface of thezinc ranges in color from a very slight iridescense to a dark blue-graydepending on the time employed in effecting the coating and the natureof the zinc alloy surface. Variations in the color of the complex-oxidecoating in no way affect its corrosion resistant properties.

Maximum corrosion resistance and paint adhesion are realized with thistreating process when a dilute acidic rinse is used following theinitial formation of the protective coating on the zinc surface. Thedilute acidic rinse may employ a dilute mineral acid rinse such as 0.01to 0.5% by weight of phosphoric or chromic acid.

When the coating compositions are to be prepared at alocation which isremote from the location where the coating operation is actually carriedout, it is desirable to supply the composition in dry form to saveshipping expense. Dry formulations according to the invention maycomprise from 1 to 500 parts by weight of an inorganic water solublecompound; from 05-200 parts by weight of a water soluble salt of analkanolamine; and from 0.001 .to 50 parts by weight of a salt of a metalioniselected from the group consisting of cadmium, manganese, aluminum,tin, titanium, molybdenum, chromium, tungsten, cobalt, ferrous andferric iron, and nickel.

A preferred dry mixture comprises 30.0 grams of sodium hydroxide, 3.9grams of triethanolamine hydrochloride, 0.25 grams of ferric nitratenonahydrate and 0.29 grams of cobalt nitrate hexahydrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following Examplesillustrate the invention. They are set forth as a further descriptionbut are not to be construed as limiting the invention thereto.

EXAMPLE I 30.0 grams of sodium hydroxide, 3.9 grams of triethanolaminehydrochloride, 0.25 grams of ferric nitrate nonahydrate and 0.29 gramsof cobalt nitrate hexahydrate were dissolved in sufficient water toresult in a total volume of one liter, and had a final composition of2.94% as sodium hydroxide, 0.39% as triethanolamine hydrochloride,0.0034% as ferric ion, and 0.0059% as cobalt ion. The resulting solutionhad a pH of 12.2.

Clean test panels of both electrogalvanize and hotdip zinc were immersedin the above aqueous alkaline solution at 120F. for a total contact timeof about 5 seconds, the panels were then withdrawn and rinsed in cleancold water for about 10 seconds and then immersed in a dilute chromicacid rinse containing about 0.1% hexavalent chromium and 0.1% phosphoricacid and with a pH of 2.5-. Temperature of the chromic acid rinse was120F. and total contact time with the acidic solution was about 5seconds. The panels were then dried in a hot air stream and visuallyinspected. It was found that a uniform light blue-gray coating wasformed on the panels which resisted mild abrasion without exposing theoriginal zinc surface. The treated panels were then painted in aconventional painting system and allowed to dry after which they weresubjected to a standard ASTM 5% salt spray and humidity testingprocedures. The results of the tests are reported in Table I.

Results of these tests shown in Table I indicated that a very highdegree of corrosion resistance, both in salt spray and humidityconditions, was afforded the zinc panels. Furthermore, additional testswere performed to determine the degree of paint adhesion for thesepanels as processed above, and it was also found that a comparabledegree of paint adhesion was afforded to the test panels afterprocessing in the aqueous alkaline treating solution.

EXAMPLE II An aqueous alkaline solution was prepared in accordance withExample I to contain 0.10% sodium hydroxide, 0.1% diethanolaminehydrochloride, a nonionic wetting agent consisting of a propyleneglycol/propylene oxide condensate, 0.0049% nickel ion added asnickelnitrate hexahydrate, and 0.005 8% molybdenum ion added as sodiummoblydate dihydrate. The resulting solution had a pH of 1 1.0.

Clean electrogalvanize and hot-dip zinc test panels were processed as inExample I and painted using a conventional painting system. The finishedtest panels were subjected to standard salt spray, humidity, deformationand paint adhesion test methods. Results given in Table I showed that avery high degree of corrosion resistance and paint adhesion was affordedby the coating produced by the solution of this invention.

EXAMPLE III An aqueous solution was prepared in accordance with Example1 to contain 2.94% sodium hydroxide, 0.39% monoethanolaminehydrochloride, 0.0025% trivalent chromium ion added as chromium nitratenonahydrate, 0.0059% cobalt ion added as cobalt nitrate hexahydrateQTheresulting solution had a pH of 12.2.

Pieces of Zamak No. 3 alloy were processed as in Example I and paintedusing a conventional painting system. The finished articles weresubjected to standards'aIt spray humidity, deformation and paintadhesion test methods. Results shown in Table I indicate a very highdegree of corrosion resistance and paint adhesion,- was afforded by thecoating produced by the solution of this invention.

EXAMPLE IV processed as inExample I and painted using a conven-' tionalpainting system. The-finished test panels were subjected to standardsalt spray, humidity, deformation and paint adhesion test methods.Results shown in Table I indicate a very high degree of corrosionresistance and paint adhesion was afforded by the coating produced bythe solution of this invention.

EXAMPLE v TABLE I Salt Spray Panel Series 500 Hours* Humidity**Adhesion*** Example 1 0-2 F 9-10 Example 11 l-3 VF 8-9 9 Example lll l-2VF 9 10-9 Example lV O-l F 9-10 8-9 Example V 1-3 VF 9 9 The panels arerated on a numerical basis from 0 to 10, with 0 representing no creepageof the corrosion from the cross scribe; where corrosion occurs, thedegree of corrosion is rated in terms of sixteenths of an inch ofcreepage from the scribed line.

"The degree of blistering in the humidity test was rated in accordancewith' the standard method of ASTM D7l4-S6, in which the size andfrequency of blisters is evaluated on a numerical scale from 10 to 0, inwhich 10 represents no blistering and the size of the blisters increasesas the numbers decrease. The frequency of the blisters is evaluated byletters in which D represents dense, MD medium dense; M representsmedium, FM represents few medium, F represents few and VF representsvery few. "The knife adhesion test comprises manually drawing a knifeblade across the surface of the coating and comparing the resistance ofdislodgement of the coating. The difficulty of dislodgement is rated innumerical values, from 10 to 0, with 10 representing excellent, 8 good,6 fair, 4 poor, 2 very poor, and 0 representing complete loss ofadhesion. I

Obviously, other modifications and variations of the present inventionare possible in the light of the above teachings. It is therefore to beunderstood that changes may be made in the particular embodiments of theinvention described which are within the full intended scope of theinvention as defined by the appended claims.

What is claimed is:

l. A method for forming a protective coating on a zinc metal surfacewhich comprises contacting said surface with an aqueous solution whichconsists essentially of an inorganic water soluble alkaline compound, atleast one metal ion selected from the group consisting of cadmium,manganese, aluminum, tin, titanium, molybdenum, chromium, tungsten,cobalt, ferrous and ferric iron and, nickel and triethanolaminehydrochloride in an amount sufficient to hold said metal ions insolution.

2. The method of claim 1 wherein said inorganic water soluble alkalinecompound is an alkali metal hydroxide.

3. The method of claim 1 wherein said aqueous solution contains bothcobalt and ferric iron ions.

4. The method of claim 1 wherein said aqueous solution is applied at atemperature between room temperature and the boiling temperature of saidaqueous solution.

5. The method of claim 1 wherein said aqueous solution has a pH above7.5.

6. An aqueous composition for the formation of a protective coating onthe surface of a zinc metal surface which consists essentially of:

a. an inorganic water soluble alkaline compound;

b. at least one metal ion selected from the group consisting of cadmium,manganese, aluminum, tin, titanium, molybdenum, chromium, tungsten,cobalt, ferrous, ferric and nickel; and

c. triethanolamine hydrochloride in an amount sufficient to maintainsaid metal ions in solution.

7. The composition of claim 6 wherein said inorganic water solublealkaline compound is an alkali metal hydroxide.

8. The composition of claim 6 wherein said aqueous solution containsboth cobalt and ferric iron ions.

9. An aqueous composition for the formation of a protective coating onthe surface of a zinc metal surface which consists essentially of water,an alkali metal hydroxide, ferric ions, cobalt ions and triethanolaminehydrochloride.

1 0. A dry mixture, for the formation of a protective coating on thesurface of a zinc metal by dissolving in water, which consistsessentially of sodium hydroxide, at least one soluble metal saltselected from the group consisting of cadmium, manganese, aluminum, tin,titanium, molybdenum, chromium, tungsten, cobalt,

ferrous, and ferric iron, and nickel; and triethanolamine hydrochloride.

1 l. The mixture of claim 10 in which, the water soluble metal salts arecobalt and ferric iron, and the water soluble salt of an alkanolamine istriethanolamine hydrochloride.

12. A dry mixture for the formation of a protective coating on thesurface of zinc metal by dissolving in a liter of water the compositionconsisting of 30.0 grams of sodium hydroxide, 3.9 grams oftriethanolamine hydrochloride, 0.25 grams of ferric. nitratenonahydrate, and 0.29 grams of cobalt nitrate hexahydrate.

13. A method for forming a protective coating on a zinc metal surfacewhich comprises contacting said surface with an aqueous solution whichconsists essentially of an inorganic water soluble alkaline compound, atleast one metal ion selected from the group consisting of cadmium,manganese, aluminum, tin, titanium, molybdenum, chromium, tungsten,cobalt, ferrous and ferric iron and nickel, and triethanolaminehydrochloride in an amount sufficient to hold said metal ions insolution.

1. A METHOD FOR FORMING A PROTECTIVE COATING ON A ZINC METAL SURFACEWHICH COMPRISES CONTACTING SAID SURFACE WITH AN AQUEOUS SOLUTION WHICHCONSISTS ESSENTIALLY OF AN INORGANIC WATER SOLUBLE ALKALINE COMPOUND, ATLEAST ONE METAL ION SELECTED FROM THE GROUP CONSISTING OF CADMIUM,MANGANESE, ALUMINUM, TIN, TITANIUM, MOLYBDENUM, CHROMIUM, TUNGSTEN,COBALT, FERROUS AND FERRIC IRON AND, NICKEL AND TRIETHANOLAMINEHYDROCHLORIDE IN AN AMOUNT SUFFICIENT TO HOLD SAID METAL IONS INSOLUTION.
 2. The method of claim 1 wherein said inorganic water solublealkaline compound is an alkali metal hydroxide.
 3. The method of claim 1wherein said aqueous solution contains both cobalt and ferric iron ions.4. The method of claim 1 wherein said aqueous solution is applied at atemperature between room temperature and the boiling temperature of saidaqueous solution.
 5. The method of claim 1 wherein said aqueous solutionhas a pH above 7.5.
 6. An aqueous composition for the formation of aprotective coating on the surface of a zinc metal surface which consistsessentially of: a. an inorganic water soluble alkaline compound; b. atleast one metal ion selected from the group consisting of cadmium,manganese, aluminum, tin, titanium, molybdenum, chromium, tungsten,cobalt, ferrous, ferric and nickel; and c. triethanolamine hydrochloridein an amount sufficient to maintain said metal ions in solution.
 7. Thecomposition of claim 6 wherein said inorganic water soluble alkalinecompound is an alkali metal hydroxide.
 8. The composition of claim 6wherein said aqueous solution contains both cobalt and ferric iron ions.9. An aqueous composition for the formation of a protective coating onthe surface of a zinc metal surface which consists essentially of water,an alkali metal hydroxide, ferric ions, cobalt ions and triethanolaminehydrochloride.
 10. A dry mixture, for the formation of a protectivecoating on the surface of a zinc metal by dissolving in water, whichconsists essentially of sodium hydroxide, at least one soluble metalsalt selected from the group consisting of cadmium, manganese, aluminum,tin, titanium, molybdenum, chromium, tungsten, cobalt, ferrous, andferric iron, and nickel; and triethanolamine hydrochloride.
 11. Themixture of claim 10 in which, the water soluble metal salts are cobaltand ferric iron, and the water soluble salt of an alkanolamine istriethanolamine hydrochloride.
 12. A dry mixture for the formation of aprotective coating on the surface of zinc metal by dissolving in a literof water the composition consisting of 30.0 grams of sodium hydroxide,3.9 grams of triethanolamine hydrochloride, 0.25 grams of ferric nitratenonahydrate, and 0.29 grams of cobalt nitrate hexahydrate.
 13. A methodfor forming a protective coating on a zinc metal surface which comprisescontacting said surface with an aqueous solution which consistsessentially of an inorganic water soluble alkaline compound, at leastone metal ion selected from the group consisting of cadmium, manganese,Aluminum, tin, titanium, molybdenum, chromium, tungsten, cobalt, ferrousand ferric iron and nickel, and triethanolamine hydrochloride in anamount sufficient to hold said metal ions in solution.